1. Field of the Invention
This invention relates to apparatus for controlling the passage of ions into a quadrupole type mass analyzer.
2. Description of the Prior Art
Quadrupole mass analyzers are well known to those desiring to analyze ions such as may be sputtered from solid surfaces to allow determination of the composition of the surfaces and as may be produced upon electron impact on residual gases to allow determination of such gases. The use of such analyzers in secondary ion mass spectroscopy is disclosed by R. Schubert and J. C. Tracy, "A Simple, Inexpensive SIMS Apparatus", Rev. Sci. Instr. Vol. 44, pp 487-491, (1973). Those workers noted that quadrupole mass analyzers could most effectively mass analyze those secondary, i.e., sputtered ions having an energy less than about 4 eV, and that in the process of producing such low energy sputtered ions, a variety of particles including fast neutral atoms, x-rays, fast metastable ions and electrons were also produced. In order to prevent these particles from confounding the analysis of the low energy sputtered ions, they found it desirable to use a 180.degree. segment of a cylindrical mirror analyzer as a prefilter in front of the quadrupole analyzer. The mirror analyzer was positioned to have its axis at an angle with respect to that of the quadrupole analyzer, such that a predominate fraction of the ions passing through the second slit in the inner cylinder were in line with the axis of the quadrupole analyzer while fast neutral atoms, high energy ions, etc. produced at the sample surface were prevented from passing directly down the axis of the cylindrical mass analyzer and into the quadrupole analyzer. Such a combination of elements is cumbersome and difficult to align. Further, the transmission coefficient of cylindrical mirror analyzers is low because they pass such a narrow band of energies and have relatively small acceptance angles.
As an adjunct to such secondary ion mass analysis it is often desirable to determine the composition of residual gases by ionizing the gases and passing the ions into the quadrupole analyzer. Accordingly, electron impact ionization chambers are known to provide the requisite ionization. However, prior to the present invention, the only known technique for performing the dual functions of prefiltering and of ionizing residual gases was a tandem arrangement of a prefilter and an ionizer ahead of the quadrupole. Such a tandem arrangement was inefficient, in that the arrangement requires additional electrical vacuum feedthroughs and special attention to prevent the possible deleterious interaction of the two assemblies, and in that the two assemblies themselves were required.